Steel cords with twisted steel filaments are known in the art, particularly in the art of rubber reinforcement, and more particularly in the art of tire reinforcement.
A 3+9+15 steel cord has been and still is a widely used steel cord, used amongs others, to reinforce the breaker or belt layers of truck tires.
An example of this 3+9+15 cord is following construction:                3×0.22+9×0.22+15×0.22+0.15 6.3/12.5/18/3.5 S/S/Z/S        
Notwithstanding this widely spread use, this 3+9+15 cord has a number of drawbacks.
A first drawback is that the way of manufacturing such a 3+9+15 cord is not economical. Indeed at least two to four different twisting steps are required to manufacture the final cord.
In a first step, the three core filaments must be twisted. In a second step, the nine intermediate layer filaments are twisted around the core filaments. In a third step the fifteen outer layer filaments are twisted around the intermediate layer filaments. As a fourth step, an additional filament is wrapped around the cord.
In the usual embodiments of a 3+9+15 cord, the two different twisting directions, S and Z, are used in order to reach a torsion balance in the cord. In the examples given hereabove, the three core filaments and the nine intermediate layer filaments have been twisted in the S-direction and the fifteen outer layer filaments have been twisted in the Z-direction. If a double-twisting apparatus is used in all the steps to manufacture such a cord, this means that the subsequent twisting in Z-direction of the fifteen outer filaments partially untwists the earlier given twists in S-direction. This means a loss of energy during the manufacturing and accentuates again the non-economical way of manufacturing such a 3+9+15 cord.
A second drawback is that a 3+9+15 steel cord has no full rubber penetration. As a consequence humidity may reach the individual steel filaments during use, which may drastically decrease the life time of the steel cord and of the reinforced tire.
Numerous attempts have been made to avoid the above drawbacks and to find an improved alternative for this 3+9+15 construction.
Some attempts were directed towards providing a steel cord construction which was more economical to manufacture. An example is a 3+9+15 cord where all the layers have been twisted in the same direction. Another example is a so-called 1×27 compact cord, where all filaments have been twisted in the same direction with the same twisting step. These attempts lead to more economical cords but do not solve the problem of rubber penetration.
Other attempts were directed towards providing a steel construction with an improved rubber penetration.
An example is a 3×d1+9×d2+15×d3 cord where the three core filaments have a filament diameter d1 which is greater than the filament diameter d2 of the intermediate layer filaments, and where the filament diameter d2 of the intermediate layer filaments is greater or equal to the filament diameter d3 of the outer layer filaments. The use of the thicker filaments in the center of the cord, lead to more space available for the layers and to unsaturated layers with spaces between the filaments.
Another example is 3+8+13 cord, i.e. a cord where the intermediate layer and the outer layer are no longer saturated with the maximum number of possible filaments. One or more filaments are omitted from the intermediate or outer layer and lead to spaces between the filaments so that rubber is able to penetrate.
Still another example are 3+9+15 cords where at least one filament in each layer, i.e. in the core, in the intermediate layer and in the outer layer are preformed so that they exhibit a wavy form. The wavy filament creates more space between the filament and the adjacent filaments and allows rubber to penetrate.
Following steel cord constructions are also widely used as reinforcement for the breaker or belt layer of a truck tire:                3×0.20+6×0.35        3×0.35+8×0.35.        
These constructions, however, suffer from the same drawbacks as the 3+9+15 construction. Two twisting operations are required to manufacture the cord and complete rubber penetration is not obtained.